The invention relates to a control method for controlling an exhaust aftertreatment system and an exhaust aftertreatment system.
Both carbon particulates and nitrogen oxides such as NO and NO2, also referred to as NOx, are typical emissions in the exhaust gas of diesel engines. Requirements for reducing such emissions increase, and trigger various approaches in the art to reduce emissions. In the European patent EP 1 054 722 B1 an exhaust aftertreatment system is disclosed which combines a particulate filter collecting soot and nitrogen oxides reducing catalysts in the exhaust tract. For removing soot NO2 is generated by oxidation of NO in an oxidation catalyst. Soot which is collected in a particulate 1 filter is oxidized by NO2. Residual amounts of NO and NO2 in the exhaust gas are reduced to nitrogen gas in a selective-catalytic-reduction catalyst (SCR catalyst) by injecting ammonia into the SCR catalyst. The ratio of NO2 and NO in the exhaust gas is adjusted by using an appropriate oxidation catalyst for a particular SCR catalyst. For instance, Pt/Al2O3 oxidation catalysts with different Pt contents produce different NO2/NO ratios. For a metal/zeolite SCR catalyst all NO should be oxidized to NO2, and for a rare-earth-based SCR catalyst a high NO2/NO ratio is desirable, whereas for transition-metal-based SCR catalysts gas mixtures of NO2 and NO are preferred instead of pure or mainly NO2 or NO gases.
The design of the oxidation catalyst usually has to be a compromise between an optimal passive burning of soot in the particulate filter and an optimal conversion of NO and NO2 in the SCR catalyst. For instance, at certain engine loads only an insufficient amount of NO is oxidized to NO2 resulting in that the particulate filter will be filled with soot and that the SCR catalyst's efficiency is low due to a surplus of NO. At other engine loads the NO2 formation in the oxidation catalyst will be too high resulting in a NO2 surplus into the SCR unit resulting in NO2 and N2O emissions. The exhaust gas composition varies strongly at different engine loads. The concurring processes described above yield only a narrow range of satisfying simultaneous soot oxidation and NOx conversion with respect to engine load and the resulting varying amounts of different kinds of constituents in the exhaust gas.
It is desirable to provide an improved control method for controlling an exhaust aftertreatment system for a wider range of engine loads and exhaust gas compositions. It is also desirable to provide an improved exhaust aftertreatment system which can handle the exhaust gas produced during a wide range of engine loads and exhaust gas compositions.
According to a first aspect of the invention, a control method is proposed for an exhaust aftertreatment system of an engine in which one or more constituents of the exhaust gas are oxidized in an oxidation catalyst and one or more constituents of the exhaust gas are reduced in a selective-catalytic-reduction catalyst, wherein the exhaust gas flows from the oxidation catalyst to the selective-catalytic-reduction catalyst. Instead of the expression “selective-catalytic-reduction catalyst” sometimes its abbreviation “SCR catalyst” is used in the text. The flow of the exhaust gas through the oxidation catalyst is controlled depending on a desired ratio among the constituents, wherein the exhaust gas enters the SCR catalyst with the desired ratio among the constituents; and the ratio among the constituents is established so that at a given reaction temperature in the SCR catalyst one specific chemical reaction is selected out of a group of possible chemical reactions which can take place among the constituents of the exhaust and the catalyst material in the SCR catalyst, wherein the selected specific chemical reaction has a higher probability to be performed than each single one of the other chemical reactions.
The control of the flow can be done in different ways, e.g. by using a fixed or variable bypass which circumvents the oxidation catalyst or by changing the space velocity of the exhaust gas flow in the oxidation catalyst. Generally, the space velocity in a chemical reactor design represents the relation between a volumetric flow of a feed and a reactor volume. The space velocity indicates how many reactor volumes of feed can be treated in a unit time.
Favourably, the efficiency of the selective catalytic reduction of the constituents of the exhaust gas can be optimized while at the same time good operating conditions can be provided for a particulate filter arranged between the oxidation catalyst and the SCR catalyst. The operating region where the exhaust aftertreatment system operates well can be enlarged compared to the prior art system which operates well only close to a few operating points of the engine. The method allows for an efficient exhaust aftertreatment with respect to cost, packaging and durability.
In a preferred development of the control method the control of the flow of the exhaust gas can be achieved by splitting the flow into a first portion flowing through the oxidation catalyst and a second portion flowing through a bypass line circumventing the oxidation catalyst. This can be easily done e.g. by using a controllable valve which controls the amount of exhaust gas in the bypass. Preferably, no catalytic component, particularly an oxidation catalyst is provided in the bypass line. Generally, however, a catalytic component, particularly an oxidation catalyst, can also be provided in the bypass line.
In a preferred development controlling the flow of the exhaust gas through the oxidation catalyst can be achieved by varying a flow velocity of the exhaust gas in the oxidation catalyst. This can be done by using an internal bypass inside the oxidation catalyst which allows to varying the flow distribution to the catalyst. The flow distribution may be varied by e.g. covering parts of the catalyst thus blocking catalyst against the exhaust gas, using flow guides for directing the exhaust gas and/or by opening valves that cover inlet and/or outlet ports in the oxidation catalyst. This may also be combined with a non-uniform distribution of the catalytically active material over the catalyst for further increasing the effect. Generally, an external bypass can be provided combined with the possibility to vary the space velocity of the exhaust gas flow.
Preferably, the ratio can be established in a way that the rate for the selected chemical reaction to be performed surmounts the rate for each single one of the other chemical reactions to be performed by at least a factor of 2, preferably a factor of 5, particularly preferable a factor of 10. The rate is 1/time unit (number of reactions/time unit).
Particularly, the ratio among the constituents is a ratio of NO2/NO close to 1 and preferably not exceeding 1, particularly NO2/NO=0.8±0.2, preferably NO2/NO=0.9±0.1, most preferably NO2/NO=0.95±0.05. By choosing a ratio close to 1 it is possible to trigger a fast and highly efficient chemical reaction which reduces NO as well as NO2 and NH3 to N2 gas and water in the presence of the SCR catalyst. This reaction is favourable for a wide range of exhaust gas temperatures from below 200° C. and above. Other chemical reactions are possible depending on the amount of NO2 and NO, i.e. ratio of NO2/NO, present in the SCR catalyst. These reactions, however, are typically slower and prone to competitive reactions producing N2O and the like.
According to a preferred development, the ratio among the constituents can additionally or alternatively be established depending on the amount of soot which is contained in a particulate filter arranged between the oxidation catalyst and the SCR catalyst. NO2 which is generated in the oxidation catalyst oxidizes soot trapped in the particulate filter. The amount of NO2 needed varies with the amount of soot in the particulate filter.
Advantageously, the ratio among the members of the constituents can be established depending on the amount of NO2 which is generated in the particulate filter. The particulate filter can comprise an oxidation catalyst and thus produce NO2 which adds to the NO2 generated in the oxidation catalyst.
According to a preferred further development, additionally or alternatively the ratio among the constituents is established depending on the amount of NO2 which is generated in the oxidation catalyst. The oxidation catalyst can generate NO2 for both the passive oxidation of soot in the particulate filter as well as for the selective catalytic reduction in the SCR catalyst. The NO2 generated in the particulate filter is reacting back to NO on the soot so that the amount of NO2 and NO formed in the particulate filter is strongly dependent on the condition of the particulate filter, e.g. the amount of soot and on the reaction temperature, i.e. the exhaust temperature, wherein the selected specific chemical reaction has a higher probability to be performed than each single one of the other chemical reactions.
The ratio among the constituents can additionally or alternatively be established depending on the amount of sulphur which is adsorbed in the oxidation catalyst. The oxidation catalyst absorbs sulphur at lower exhaust gas temperatures and releases the sulphur at temperatures above 350° C. If operating conditions of the engine let the oxidation catalyst adsorb a lot of sulphur contained in the exhaust gas, the NO2 formation in the oxidation catalyst will be poisoned.
Favourably additionally or alternatively, the ratio among the constituents can be established depending on the amount of ammonia which is provided in the SCR catalyst. On an SCR catalyst ammonia is reacting with NOx to form nitrogen. On vehicles urea is injected into the exhaust gas and by the exhaust temperature urea is thermolyzed and/or hydrolyzed to ammonia in the exhaust gas and on the catalyst.
Additionally or alternatively, the space velocity of the exhaust gas in the oxidation catalyst and/or the portion of exhaust gas which can be fed into the bypass line and the portion of exhaust gas which can be fed into the oxidation catalyst can be controlled depending on operating parameters of the engine and/or on operating parameters of one or more catalysts arranged in the exhaust aftertreatment system. Consequently, a NOx or NO2 sensor can be replaced by a virtual sensor which uses a model of the engine and the exhaust aftertreatment system to calculate the relevant parameters, particularly the NO2 and NO content in the exhaust gas at the inlet of the SCR catalyst. Preferably parameters are available such as exhaust gas flow, temperatures in the oxidation catalyst and particulate filter, NO and NO2 flow from the engine, soot flow from the engine and/or soot load in the particulate filter. Some of the parameters can be measured and other parameters can be calculated from other sensors and engine parameters.
According to another aspect of the invention an exhaust aftertreatment system comprising at least an oxidation catalyst and a SCR catalyst arranged in an exhaust line of an engine is proposed wherein the flow of the exhaust gas through the oxidation catalyst is controllable depending on at least one desired ratio among one or more pairs of the one or more constituents, wherein the exhaust gas enters the selective-catalytic-reduction catalyst with the at least one desired ratio among the one or more pairs of the one or more constituents; the at least one desired ratio among the one or more pairs of the one or more constituents at the input of the selective-catalytic-reduction catalyst is established; a predetermined reaction temperature or temperature range is selectable and is established in the selective-catalytic-reduction catalyst; the probability that one specific chemical reaction out of said group of possible different chemical reactions between the one or more constituents of the exhaust gas and the catalyst material in the selective-catalytic-reduction catalyst will take place is increased by inputting the exhaust gas into the selective-catalytic-reduction catalyst, wherein said reaction probability for said selected specific chemical reaction is higher than the reaction probability for each one of the other chemical reactions which are not selected.
Preferably the exhaust aftertreatment system comprises a sensing unit for controlling the flow of the exhaust gas through the oxidation catalyst depending on at least one desired ratio among one or more pairs of the one or more constituents, wherein the exhaust gas enters the selective-catalytic-reduction catalyst with the at least one desired ratio among the one or more pairs of the one or more constituents; and at least one unit coupled to the oxidation catalyst and/or a particulate filter for establishing at least one desired ratio among the one or more pairs of the one or more constituents at the input of the selective-catalytic-reduction catalyst, wherein a predetermined reaction temperature or temperature range is selected and established in the selective-catalytic-reduction catalyst.
Said flow control through the oxidation catalyst can be achieved by an external bypass line which circumvents the oxidation catalyst. A controllable valve can favourably vary the portion of exhaust gas flowing through the oxidation catalyst and generating an oxide, e.g. NO2, and the portion of exhaust gas flowing through the bypass line. Generally, the bypass line can also be provided with an oxidation catalyst, e.g. a smaller or less efficient one, so that the oxide generation is mainly performed in the bypassed main oxidation catalyst.
Alternatively or additionally, the flow control can be achieved by controlling the space velocity of the exhaust gas flowing through the oxidation catalyst. Preferably one or more closing units can be provided which close or open channels or areas in the oxidation catalyst thus reducing or increasing the catalyst volume accessible for the exhaust gas and hence increasing or decreasing the space velocity. An increase in space velocity results in a decrease in oxidized matter and a decrease in space velocity results in an increase of oxidized matter.
Preferably a particulate filter can be arranged in between the oxidation catalyst and the SCR catalyst. Soot trapped in the particulate filter can be oxidized by the oxidized matter, particularly NO2, generated in the oxidation catalyst.
A sensing unit can be provided for sensing the amount of NO2 contained in the exhaust entering the SCR catalyst. The sensing unit can comprise a control unit for controlling a valve of a bypass line external to the oxidation catalyst and/or for controlling one or more closing units in the oxidation catalyst for opening or closing parts of the oxidation catalyst and consequently varying the space velocity in the oxidation catalyst. The sensing unit can favourably comprise a NO2-sensitive sensor arranged in the exhaust line downstream of the particulate filter. Optionally, the sensing unit can comprise a device which calculates the amount of NO2 and/or the ratio of NO2/NO entering the SCR catalyst depending on operating parameters of the engine and/or on operating parameters of one or more catalysts arranged in the exhaust aftertreatment system, thus providing a virtual sensor.
According to another aspect of the invention a computer program storable on a computer readable medium, comprising a program code for use in a method comprising at least the steps of: (a) controlling the flow of exhaust gas through an oxidation catalyst depending on at least one desired ratio among one or more pairs of the one or more constituents, wherein the exhaust gas enters a selective-catalytic-reduction catalyst (70) with the at least one desired ratio among the one or more pairs of the one or more constituents; (b) establishing the at least one desired ratio among the one or more pairs of the one or more constituents at an input of the selective-catalytic-reduction catalyst; (c) selecting a predetermined reaction temperature or temperature range and establishing it in the selective-catalytic-reduction catalyst; (d) increasing the probability that one specific chemical reaction out of said group of possible different chemical reactions between the one or more constituents of the exhaust gas and the catalyst material in the selective-catalytic-reduction catalyst will take place by inputting the exhaust gas into the selective-catalytic-reduction catalyst, wherein said reaction probability for said selected specific chemical reaction is higher than the reaction probability for each one of the other chemical reactions which are not selected.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.